This invention is directed to an improved plastic film having excellent antistatic properties and to a method for producing such film. More specifically, the invention concerns improved biaxially oriented polyester films having improved antistatic properties coupled with altered surface properties which render the film printable or coatable with additional coatings such as photosensitive compositions.
The growth of the packaging, reprographic, microfilm and magnetic tape markets has significantly increased the utilization of plastic film materials in these fields, particularly biaxially oriented plastic film made from polyethylene, polypropylene, nylon, polyester, polystyrene and polycarbonate. One of the problems associated with the use of such film is the tendency of these materials to accumulate an electrostatic charge on the surface as the result of handling or processing of the film, particularly at low relative humidity. This electrostatic charge attracts not only dust and other contaminants, but additionally attracts other film. For instance, in the case of polyester film supports, electrostatic charges may interfere with the collating, sorting and developing of microfiche cards or transparent supports and cause these materials to stick together.
To overcome this serious problem, antistatic films have been developed. Antistatic films are usually formed by the application of an antistatic coating onto the surface of the film. Many of these coatings have successfully reduced the electrostatic properties of the film to satisfactorily low levels. However, many of these coatings can cause blocking. That is, the film coated with an antistatic layer has a coefficient of friction high enough so that one layer of the film does not slip over another or over rollers and other equipment over which the film moves during operation.
Another problem arising from the coating of an antistatic layer onto a surface of a film is the decreased clarity resulting therefrom. This property, usually reported as haze, is very important in microfilm applications. Microfilm must be readable. Light is projected through the film so that the viewer may read the information printed on the film. Poor clarity results in fuzziness and difficulty in reading because of loss of light intensity.
Yet a third problem associated with the application of an antistat coating is its ability to remain adhered to the surface of the film, particularly where the film is later treated with printing or coating compositions.
Also important to the film manufacturer is the reclaimability of scrap film produced during the film making process. Any coating applied during manufacture will be present on the scrap film as an impurity. This impurity can give rise to a discoloration as a consequence of film reprocessing because of high temperature conditions encountered in the film making process.
Various antistat coatings are disclosed in the prior art. For example, U.S. Pat. No. 4,214,035 teaches the application to polyester film of an antistat coating based on a mixture of stearamidopropyldimethyl-beta-hydroxyethylammonium nitrate and a crosslinkable acrylic terpolymer binder, present in an approximately three to one ratio respectively. Although this antistat coating improves the antistat properties of the film, the relatively high level of antistat compound to binder renders the coating relatively more expensive. Also, reclaim from such film does exhibit discoloration when reprocessed into new film which is undesirable, particularly where the film is to be used for reprographic applications. Finally, the antistat surface of such film is not readily susceptible to the application of additional coatings, particularly in reprographic applications.
British Pat. No. 1,558,064 discloses the use of certain quarternary ammonium salts as an antistat coating for polyolefin film such as polypropylene. These salts have the formula: ##STR2## wherein A is lower alkyl, R is an aliphatic radical having 1 to 22 carbons, x and y are integers having the sum of 2 to 5, and X.sup.- is an anion. The utilization of this material as disclosed in the patent, aside from its effectiveness as an antistat, does not resolve the problem of the lack of adhesion of additional coatings over the antistat coating.
Accordingly, it is an object of this invention to provide plastic film material having improved antistatic properties as well as good blocking properties and low haze.
Another object is to provide a method for improving the antistatic properties of biaxially oriented polyester film for use in the graphic arts field.
Still another object is to provide an antistat coating on at least one surface of biaxially oriented polyethylene terephthalate film which coating also provides a suitable base coat or primer coat for the application of additional coatings.
Yet another object is to provide a biaxially oriented plastic film material having enhanced antistatic properties and which can be successfully reclaimed in the plant by the film manufacturer.